This invention relates generally to absorption refrigeration systems. More particularly the invention relates to corrosion inhibiting working fluids for use in such systems and to a method of manufacturing such fluids. The fluids have corrosion inhibitors that reduce the corrosion of metals in the system that are in contact with the system working fluid.
There are a number of different pairs of refrigerants and absorbents that are used in absorption refrigeration systems. One such pair is water and a halogen salt of lithium, such as lithium bromide, lithium chloride or lithium iodide, mixed to form a concentrated aqueous solution of the lithium salt. Another such pair is water and ammonia.
Iron and iron alloys like steel and copper and copper alloys are typical construction materials used in absorption refrigeration systems. Corrosion of these materials can cause difficulties. Not only is metal loss of concern but also the oxidation of ferrous metal produces gaseous hydrogen. If not purged, the hydrogen can interfere with the proper operation of the system. Corrosion is of particular concern in systems that use halogen salts of lithium. And regardless of the refrigerant/absorbent pair used in a particular system, metal corrosion rates increase as system temperatures increase.
It is well known in the prior art that the addition of a salt of chromium, such as lithium chromate, to the refrigerant/absorbent solution in an absorption refrigeration system, is effective in reducing metallic corrosion. The presence of the chromate compound promotes the formation of a protective layer of iron and chromium oxides on the surfaces of the system that are in contact with the absorbent. With a reduction in iron oxidation, there is also a corresponding reduction in the production of noncondensible hydrogen. There is some concern, however, about the health risks that chromium presents. At least one government authority, the U.S. Environmental Protection Agency, has identified chromium as a carcinogen, and has prohibited the presence of chromium compounds in systems that are open to the atmosphere. As of this writing, there is no restriction on the use of chromium compounds in closed systems but there may be such a restriction in the near future. Absorption refrigeration systems are, of course, closed systems, but a certain amount of working fluid from the system can become exposed to the atmosphere through the taking of samples, the manufacturing process and spills during handling and filling. And, at the end of the service life of a system, the system charge will necessarily require disposal of the working fluid, including the chromium compounds that it contains.
What is needed are working fluids for absorption refrigeration system that have an inhibitor that is effective in reducing the corrosion of metallic components of the system and that is free of the environmental and health drawbacks of inhibitors that contain compounds of chromium. The fluids should be adaptable to a variety of different system types and be effective at inhibiting corrosion even at the very high temperatures experienced in triple effect absorption systems and in systems fired by natural gas or propane.